1. Field of the Invention
This invention relates generally to wireless networks, and, more particularly, to location tracking of wireless network mobile units in a multi-path environment.
2. Description of the Related Art
A wireless Local Area Network (LAN) is a flexible data communications system that can either replace or extend a traditional, wired LAN to provide added functionality. A traditional, wired LAN sends data packets from one piece of equipment to another across cables or wires. Instead of the wires used in wired LANs, a wireless LAN relies upon radio waves to transfer data between one or more fixed or mobile units and one or more access points. Data is superimposed onto the radio wave through a process called modulation, and the modulated radio wave then acts as the transmission medium. Wireless LANs are used in various vertical and horizontal applications (e.g., retail, manufacturing, logistics, healthcare, education, public space, etc.). Recently, there has been a surge in the deployment of wireless infrastructure networks to provide wireless internet access services, especially in public “hot spots” covering airports, hotels, coffee shops, and the like.
The ability to track the locations of the mobile devices may increase the utility of a wireless LAN. For example, the wireless LAN may be able to provide location-based services, such as phone calls, e-mails, location-dependent information, and the like, to mobile subscribers as they move between and within different locations, e.g. around the interior of a warehouse, from home to the office, from indoors to outdoors, and the like. For another example, location tracking may be used to manage assets in warehouses, hospitals, docking yards, and the like, by fixing radio frequency tags to mobile assets and thereby making them “visible” to people and information systems via the wireless LAN.
One conventional technique for tracking mobile devices and/or radio frequency tags, often referred to as a Received Signal Strength Indicator (RSSI) method, uses measurements of power received from the mobile device and/or radio frequency tag at several access points to estimate the location of the mobile device and/or radio frequency tag. A second conventional technique for tracking mobile devices and/or radio frequency tags, often referred to as a Time Difference Of Arrival (TDOA) method, measures the arrival times of signals transmitted between the mobile device and/or radio frequency tag and several access points, and then uses triangulation to estimate the location of the mobile device and/or radio frequency tag. However, both the RSSI and TDOA methods suffer significant degradation in accuracy and/or overall system performance when they are used to track locations mobile devices and/or radio frequency tags in rich multi-path environments.